The spent nuclear reactor fuel elements are loaded into a transport container for transporting the same to a storage area. These transport containers must be closed so as to be gas-tight and this is achieved with a cover sealing system. Also, the transport containers must provide adequate shielding against radioactivity. In addition, the transport containers must have an adequate mechanical strength which can also resist accident conditions. Further, the transport container has to be so configured that the heat of radioactive decay can be safely conducted to the outside.
The loaded transport containers are transferred to a temporary storage facility where they are kept until the irradiated nuclear reactor fuel elements are reprocessed or until they are put away for long-term storage or direct final storage. The transport containers must then be opened again. In the event that a direct final storage is decided upon, the spent nuclear reactor fuel elements must be packed in special final storage containers and, in these containers, the nuclear reactor fuel elements are transferred to geological formations for safe final storage.
The final storage containers just have certain final storage characteristics. Such containers must be mechanically stable, corrosion-resistant and tightly sealed. The vessel of the final storage container is therefore made of steel or cast iron in order to guarantee the mechanical stability of the container. It is preferable to use cast iron with nodular graphite (GGG-40) for the thick-walled container vessels because spheroidal cast iron exhibits high strength and toughness. The grade GGG-40 is listed in German Nodular cast iron specifications.
Since the corrosion resistance of steel or cast iron with nodular graphite is inadequate for the purpose of final storage, it has been suggested to apply a corrosion-resistant protective layer to the outside of the container vessel made of steel or cast iron. This corrosion-resistant protective layer can be made of ceramic, graphite or other material.
In the copending application entitled "A Container for the Interim and Long-term Storage of Radioactive Material" having Ser. No. 451,934 and filed on Dec. 21, 1982, it is disclosed that a container for the final storage of irradiated nuclear reactor fuel elements can be made so that it has a vessel having two metal wall layers wherein the inner wall layer is made of a mechanically stable, inexpensive material and the outer wall layer is made of a corrosion-resistant material. The inner layer is made of cast iron with nodular graphite or laminar graphite and the outer layer is made of a high-alloy austenitic castable material with nodular graphite. The outer layer is cast in surrounding relationship to the inner layer. The receiving opening of the container is closed by means of a sealing cover welded to the outer layer.
The fuel element container configured as described above can be also utilized for a longer term storage of the irradiated fuel at a surface location and for transporting the same if the thickness of the inexpensive inner layer is increased to correspond to the requirements for shielding. However, a consequence of this arrangement is that the costly corrosion protective wall layer, which must have a pre-determined thickness in order to be adequate for the final storage, is applied to an inner wall layer of a container vessel having a larger diameter. The size and therefore the amount of material required for the corrosion protective layer is increased.